1. Field of the Invention
The present invention relates to an improved planarization coating layer comprising a selected novolak resin. The present invention also relates to a process of planarizing topographical features on a substrate. Furthermore, the present invention relates to a multilayer structure suitable for lithographic applications comprising a substrate having topographical features, a layer of selected planarizing coating material thereover which comprises a selected novolak resin, and an optional layer of photoresist material overlying the layer of planarizing material.
2. Brief Description of Prior Art
Multilayer resist imaging is becoming increasingly important for patterning submicron geometries in electronic devices. Using this technique in a bi-layer imaging system, the wafer topography, which may consist of epitaxial silicon islands, is covered by a layer of polymer to provide a relatively flat surface. This polymer layer is commonly called a planarization layer. A thin layer of photo or electron-beam resist is coated over this surface and patterned. Because the imaging overlayer is thin and flat, its pattern resolution is excellent and transferable to the wafer substrate by anisotropic plasma etching.
In a trilayer imaging system, a thin intermediate layer of oxygen-plasma resistant material, such as silicon oxide or nitride hardmask layer, is applied between the planarization layer and imaging resist layer. With such tri-layer systems, the upper resist layer is first exposed to radiation and then wet developed to create a pattern. During this operation, the intermediate silicon oxide or nitride hardmask layer and the lower planarization layers are unaffected. Next, the intermediate layer is dry patterned by conventional methods. Finally, the planarization sublayer is dry developed with an oxygen plasma by conventional methods.
Choosing the right material for a planarization layer is not an easy job. Planarization layers for submicron pattern imaging must have particularly flat upper surfaces to maintain precise line width control.
Generally, there are two types of planarization layers used today. One type is nonremovable material. This type is used principally as an insulating material. Polyimides and spin-on glass are typical material used for this type of planarization layer. The second type is removable and reflowable materials. Typical classes of this material include poly(methyl methacrylate) (PMMA),novolaks, photoresist compositions, and polyesters. U.S. Pat. No. 4,621,042, which issued to Pampalone et al. on Nov. 4, 1986, teaches the use of Pure o-cresol novolak resin as a planarization layer. This patent is incorporated herein by reference in its entirety.
In early semiconductor multilayer lithographic processes, commercial photoresists made with a high percentage of novolak resins, particularly m-cresol novolaks were used as planarization processes. In 1986, T. R. Pampalone, J. J. DiPiazza, and D. P. Kanen (see "Novolak Resin Planarization Layers for Multilayer Resist Imaging Systems" Journal of the Electrochemical Society, Vol. 133, No. 11, November 1986, pages 2394-2398 and U.S. Pat. No. 4,621,042) suggested that photoresist compositions are a poor choice for a planarization layer material. Instead, they urged that novolak resins which can flow significantly before the onset of polymerization make the best planarization layers. This happens when the resin melts at low temperatures and polymerizes at high temperature (i.e., reacts slowly). They suggested that o-cresol novolak resins have this characteristic.
Pampalone et al. also stated that other important parameters to be considered in choosing a material for a planarization layer include its ability to accept the deposition of hardmask materials and imaging resist materials over it as well as its susceptibility to oxygen RIE plasmas.
While pure o-cresol novolak resins are suitable materials for some applications, they form unacceptable gels and pinholes when used as a reflowable planarization coating in some microlithographic processes. Also, low molecular forms of o-cresol novolak resins have unacceptable adhesion properties.
The present invention is directed toward an improved certain novolak planarization material which does not have the gel and pinhole problems which are associated with pure o-cresol novolak resin.
3. Citation of Possibly Relevant Art
The following U.S. patents and literature are cited to further illustrate some of the various materials which have been employed for planarization materials.
U.S. Pat. No. 4,362,809, which issued to Chen et al. on Dec. 7, 1982, teaches the use of a dried polymethylmethacrylate (PMMA) resin as a planarization layer.
U.S. Pat. No. 4,427,713, which issued to White et al. on Jan. 24, 1984, teaches a photoresist containing a novolak resin and a diazoquinone sensitizer as a planarization layer.
U.S. Pat. No. 4,515,828, which issued to Economy et al. on May 7, 1985, teaches that diacetylenic prepolymers, novolaks, and low molecular weight precursors of epoxies and polyimides may be used as planarization layers.
U.S. Pat. No. 4,532,005, which issued to Grieco et al. on Jul. 30, 1985, teaches that a novolak-type photoresist could be used as a bottom or planarization layer in a tri-layer resist system.
U.S. Pat. No. 4,612,210, which issued to Hofer et al. on Sep. 16, 1986, teaches the use of polyamide alkyl esters as a planarization layer.
U.S. Pat. No. 4,621,042, which issued to Pampalone et al. on Nov. 4, 1986, teaches the use of essentially pure o-cresol novolak resins as a planarization layer.
U.S. Pat. No. 4,642,162, which issued to Brownell et al. on Feb. 10, 1987, teaches the use of an organic material (e.g., positive photoresists and polyimides) as planarization layer materials.
U.S. Pat. No. 4,654,113, which issued to Tuchiya et al. on Mar. 31, 1987, teaches various organic and inorganic materials may be used as planarization layer. See col 2, lines 30-36.
U.S. Pat. No. 4,665,007, which issued to Cservak et al. on May 12, 1987, teaches that a positive diazo ketone novolak photoresist may be used as a planarization layer.
U.S. Patent Nos. 4,702,993 and 4,810,617, which issued to White et al. on Oct. 27, 1987 and Mar. 7, 1989, teach polyimides and certain positive resists are suitable as a planarization layer in multilayer electron beam resists.
U.S. Pat. No. 4,732,847, which issued to Radigan on Mar. 22, 1988, teaches resins such as photoresists, polyimides, polymethacrylates, and the like in a tri-level resist system.
U.S. Pat. No. 4,741,926, which issued to White et al. on May 3, 1988, teaches the use of PMMA and novolak resin/diazoquinone sensitizer photoresist composition as a planarization layer.
U.S. Pat. No. 4,745,045, which issued to Fredericks et al. on May 17, 1988, teaches that a mixture of a positive photoresist and polyamic acid (which converts to a polyimide) may be used as a planarization layer.
U.S. Pat. No. 4,803,147, which issued to Mueller et al. on Feb. 7, 1989, teaches that certain solvent-soluble polyimide polymers may be used as planarization layers.
U.S. Pat. No. 4,806,453, which issued to Vidusek et al. on Feb. 21, 1989, teaches that a positive polyglutarimide photoresist may be useful as a planarization layer.
D. V. Comello, "Planarizing Lead Edge Devices" Semiconductor International November, 1990, pages 60-62, 64, and 66 provide a synopsis of various planarization materials and techniques used today in the semiconductor industry.
Separately, the following U.S. Patents are cited because they teach certain phenolic novolak resin compositions and their use in photoresist formulations.
U.S. Pat. No. 4,123,279, which issued to Kobayashi on Oct. 31, 1978, teaches a photoresist formulation containing a Phenol-aldehyde novolak resist made by reacting a substituted phenol, preferably t-butylphenol, with an aldehyde under acid conditions.
U.S. Pat. No. 4,173,473, which issued to Fabrenholtz et al. on Nov. 6, 1979, teaches making a novolak resin by reacting a cresol, an aldehyde or reactive ketone, and an aromatic hydroxyl compound having an alkyl side chain of from 3-15 carbon atoms. The only specifically mentioned aromatic hydroxyl compound is 2-tertiary butyl phenol
U.S. Pat. No. 4,308,368, which issued to Kubo et al. on Dec. 29, 1981, teaches novolak resins made by reacting a mixture of selected substituted phenols (e.g., t-butylphenol, n-butylphenol), phenol, and or cresol with an aldehyde.